In 1979, Perry published "The Problem of the Essential Indexical" in which he combined his work on philosophy of language and philosophy of mind. Essential indexicals (I, here, and now) are parts of language that cannot be paraphrased away. They are seen as locating beliefs and are essential to understand the speaker's belief. Perry presents a now famous example to illustrate his point:

"I once followed a trail of sugar on a supermarket floor, pushing my cart down the aisle on one side of a tall counter and back the aisle on the other, seeking the shopper with the torn sack to tell him he was making a mess. With each trip around the counter, the trail became thicker. But I seemed unable to catch up. Finally it dawned on me. I was the shopper I was trying to catch."

In this example, the pronoun "I" is essentially indexical because it allowed Perry to realize that it was he himself making the mess. This realization caused him to change his behavior. Essential indexicals create the impetus for action. They cannot be paraphrased away while retaining their immediacy. If Perry were to say "Perry realized that Perry was making a mess," it would still not be essentially indexical because Perry would still have to understand that he himself is Perry. Without that extra step, there would be no reason for him to change his action. "I" is the only essential indexical in that situation.

In logic, Perry and Jon Barwise are known for discussion of the slingshot argument, especially in their 1981 article "Semantic Innocence and Uncompromising Situations".

In his 2001 book Knowledge, Possibility and Consciousness, Perry argues for what he calls "antecedent physicalism", according to which physicalism is antecedently taken to be a plausible and reasonable position, provided that there are no better rival theories. Thus, Perry defends a version of type physicalism against three major philosophical arguments for dualism: the zombie argument, the knowledge argument, and the modal argument.